Troubleshooting Convergence Problems

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Troubleshooting Convergence Problems Special
Slides for ChE 656 Prepared by Dr. Hong-ming
Ku May 10, 2003
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Troubleshooting Design-Spec Problems

• Common errors encountered
• 1. Function is not monotonic.
• Possible cure Narrow the bounds enough for the
  function to be
• monotonic.
• 2. Function is flat and not changing.
• Possible cure Examine the manipulated variable
  again and make sure
• it really does affect the objective function.

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Troubleshooting Design-Spec Problems (Contd)

• 3. Solution is outside the bounds.
• Possible cure Increase the bounds.
• 4. Secant method fails to converge in 30
  iterations.
• Possible cure Dont bother increasing the number
  of iterations.
• Instead, switch to other convergence methods,
  such as Broyden.
• Important If the workaround does not work,
  replace the design-spec
• with a sensitivity block to see the trend between
  the manipulated
• variable and the objective function.

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Troubleshooting Tear Stream Problems

• The biggest problem is the tear stream loop not
  converging.
• Can be caused by
• 1. Numerical problem or instability in the
  convergence
• 2. Fundamental problems in the flowsheet caused
  by user mistakes
• For more reading on stream convergence, read the
  paper Dont Let Recycle Stream Stymie Your
  Simulations by Ryan C. Schad, Chemical
  Engineering Progress, December 1994, pp. 68-76

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Numerical Problems in Flowsheet Convergence

• Flowsheet is well defined, but the default
  algorithm cannot converge the tear streams.
• Examine the history of convergence and see if the
  Max Error/
• Tolerance slowly goes down and approaches 1.
• - If so, increase the number of iterations
  beyond the maximum of 30.
• - If not, try a another algorithm, such as
  Broyden.
• - If design-spec exists, try nesting the tear
  stream loop inside the design-spec
• loop or converge both loops simultaneously.

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Numerical Problems (Contd)
Max Error/Tolerance must drop below 1 before
there is convergence
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Common User Mistakes in Flowsheet Convergence

• Did not allow all components to leave the process
  (accumulation)

C2H4Cl2 --gt C2H3Cl HCl C2H4Cl2 --gt C2H2
2HCl C2H3Cl C2H2 --gt C4H5Cl
C2H2 HCl
C2H3Cl
C2H4Cl2
C2H4Cl2 C4H5Cl
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Common User Mistakes (Contd)

• Mass in ? Mass out

Specify the flow of Other stream (lt 7 lb/h) gt
no problem
Cannot specify the flow of Recycle So what do we
specify for x?
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Common User Mistakes (Contd)

• Solutions to avoid the above problem
• Specify the flow of the recycle instead
• Better yet, specify the output flow as a ratio of
  the inlet flow.
• Another example of Mass in ? Mass out

D 100 lbmol/hr
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Common User Mistakes (Contd)

• Its better to specify D/F instead of D for the
  first column to avoid taking out more than what
  is coming in.
• System-generated tear streams should not contain
  zero flows
• Solution provide a reasonable initial guess for
  the tear stream of choose another tear stream

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Guidelines to Avoid Convergence Problems

• First the good news most simulators have no
  trouble converging simple flowsheets with only a
  few recycle streams and design-specs
• Always provide an initial guess for the tear
  streams.
• Do not introduce recycles unnecessarily

Model using one reactor and one flash drum
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Guidelines to Avoid Convergence Problems (Contd)

• Build your flowsheet slowly, a few blocks at a
  time. Once you run into a convergence problem,
  you only have to retreat one step to
  troubleshoot.
• Start your complex model with some conditions
  that gives convergence. Then, change the
  conditions slowly without doing reinitialization.
• Start your complex model with simple modules such
  as separators instead of columns to get a
  converged solution first.

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Guidelines to Avoid Convergence Problems (Contd)

• Turn off energy balance if you have a complex
  model that is converging very slowly.
• In a complex flowsheet with many nested loops,
  make sure the tolerance in the inner loops are
  successively tighter than those on the outer
  loops to avoid noises in the convergence.